If some helium is mixed with the CO in the outer zone of the WD, it diffuses out. I very crudely estimate the diffusion coefficient of helium through CO matter based on expression from, e.g., [1]} and [2]}, to be \(D \approx 0.1 (T/10^8 {~\rm K})^{3/2} (\rho /10^6 {~\rm g}{~\rm cm}^{-1})^{-2/3} {~\rm cm}^2 {~\rm s}^{-1}\) .
The helium is spread in a layer of width \(\Delta r \approx 1000 {~\rm km}\) . The diffusion time will be therefore \(\tau _{\rm diff} \approx (\Delta r)^2 /D\) .
For the above parameters, and since the WD cools over this long time scale, I estimate that \(\tau _{\rm diff} > 10^9 {~\rm yr}\) . (I get the same crude timescales from scaling the tables that [3]} give).
By that time the WD has cooled. I conclude that if the helium mixture with carbon and oxygen in the hot WD merger remnant did not ignite the helium (but I do expect ignition of helium), the floating of helium on the surface of the WD is unlikely to ignite a VLTP.
